1. Field of the Invention
This invention relates to an optical pickup apparatus used for recording and reproducing an optical disc, and more particularly to an optical pickup apparatus that is capable of eliminating the crosstalk effectively.
2. Description of the Prior Art
Generally, an optical pickup apparatus irradiates a light on the recording face of an optical disc to detect the reflected light, thereby performing the information recording and reproducing operation. To this end, the optical pickup apparatus is composed of a laser diode for emitting a light, an objective lens for focusing the emitted light on the recording face of the disc, and other optical system components required for concentrating and receiving the light.
An optical disc, such as a compact disc(CD) or a digital versatile disc(DVD) having more improved recording capacity, has been commercially available. This optical disc has wider use for recording and reproducing audio and video data and computer data, etc. Recently, an optical disc having much more improved recording capacity is expected owing to the development of a blue laser generating a short wavelength of light. There have been suggested several schemes for increasing the numerical aperture(NA) of an objective lens or narrowing the track pitch along with the use of a light source such as a blue laser, etc. so as to enlarge the recording capacity of the optical disc. However, it has a problem in that cross talk is caused between the adjacent tracks when there is narrowing of the track pitch of the optical disc.
More specifically, since an optical spot irradiated so as to reproduce a certain pit P1, it has a greater width than a width of the pit shown in FIG. 1, and it is also irradiated onto the pits in the adjacent tracks. As a result, a cross talk component caused by the pits in the adjacent tracks is involved in the reproduced signal. A strategy employing a polarizing phase plate as shown in FIG. 2 has been known as one of the methods used for eliminating such a cross talk component.
FIG. 2 shows the conventional optical pickup apparatus for eliminating the cross talk component. In FIG. 2, the optical pickup apparatus includes a light source 12 for generating a light beam, an objective lens 20 for focusing a light beam from the light source 12 on the recording face of an optical disc 22, first and second photo detectors 26 and 28 for converting a reflective light beam from the optical disc 22 into an electrical signal, a beam splitter 18 arranged among the light source 12, the objective lens 20 and the first and second photo detectors 26 and 28, a polarizing beam splitter(PSB) arranged among the beam splitter 18 and the first and second photo detector 26 and 28, a polarizing phase plate 14 arranged between the light source 12 and the beam splitter 18, and a collimator lens 16 arranged between the polarizing phase plate 14 and the beam splitter 18. The light source 12 generates two polarized beams having a polarizing characteristic moving perpendicularly to each other. The collimator lens 16 converts a divergent light beam progressing from the light source 12, via the polarizing phase plate 14, toward the beam splitter 18 into a parallel light beam to prevent leakage of the light beam. The beam splitter 18 passes a light beam from the collimator lens 16 in such a manner as to be progressed toward the objective lens 20, and reflects a reflective light beam reflected from the recording face of the optical disc 22, and passing through the objective lens 20 in such a manner as to be progressed toward the PSB 24. The objective lens 20 focuses an incident light beam from the beam splitter 18 on the recording face of the optical disc 22. The polarizing phase plate 14 consists of two phase segments(0, xcfx80) having a phase difference of 180xc2x0 in the left and right phases thereof as shown in FIG. 3A, and which is responsible for selectively changing a phase of the light beam from the light source 12 in accordance with a polarizing characteristic thereof. More specifically, the polarizing phase plate 14 passes the first polarized beam in any one direction of two polarized beams emitted from the light source 12 as it is, thereby allowing the first polarized beam to be irradiated, via the collimator lens 16, the beam splitter 18 and the objective lens, on the recording face of the optical disc 22 as a main beam as shown in FIG. 2A. On the other hand, the polarizing phase plate 14 passes the second polarized beam in a direction perpendicular to the first polarized beam of two beams from the light source 12 with a phase being modulated, thereby allowing the second polarized beam to be irradiated on the recording face of the optical disc 22 as a sub-beam having a double-mountain shape superposed at each side of the main beam as shown in FIG. 2A. The main beam in the light beams irradiated on the recording face of the optical disc 22 in this manner is irradiated on the signal track to be accessed and is used to reproduce an information signal. On the other hand, the sub-beam is irradiated on the adjacent tracks and is used to detect a cross talk component included in the reproduced signal. The PSB 24 passes the main beam in a reflective light beam reflected from the disc 22 and received via the objective lens 20 and the beam splitter 18 as it is in such a manner to be progressed toward the first photo detector 26, whereas it reflects the sub-beam in such a manner to be progressed toward the second photo detector 28. The first photo detector 26 detects a main beam received from the PSB 24 and detects a sub-beam received from the PSB 24, thereby converting them into electrical signals. In other words, the first photo detector 26 detects a radio frequency signal including an information signal from the main beam while the second photo detector 28 detects a cross talk component in the adjacent tracks from the sub-beam.
Further, the optical pickup apparatus includes an amplifier 30 connected to the second photo detector 28, and a differential amplifier 32 connected to the first photo detector 26 and the amplifier 30. The amplifier 30 amplifies and outputs a crosstalk component in the second photo detector 28, and the differential amplifier 32 eliminates and outputs a crosstalk component output from the amplifier 30 from a radio frequency signal output from the first photo detector 26.
The optical pickup apparatus must have the ability to vary a distance between the sub-beams irradiated on the adjacent tracks so as to access all of the optical discs having a different track pitch accurately. In the above mentioned optical pickup apparatus, however, a degree of freedom for its design is deteriorated because a wavelength(xcex) of a beam determining a distance between the sub-beams or the numerical aperture(NA) of the objective lens must be controlled such that the distance between the sub-beams can be controlled.
More specifically, assuming that a distance extending from the center of the double-mountain shaped sub-beam as shown in FIG. 3B into a peak thereof, that is, a position of the sub-beam should be x, x is equal to fsinxcex8(wherein f is a focus length of the objective lens, and xcex8 is an angle at which a line linking the sub-beam with the center of the objective lens makes an optical axis of the objective lens). Also, assuming that a diameter of the objective lens is a and a wavelength of a beam be xcex, asinxcex8 is equal to xcex when considering the diffraction equation. In consideration of said relationship, a position value(x) of the sub-beam can be derived from the following equation:                     X        =                              f            ⁢                          λ              a                                =                      λ                          2              ⁢              NA                                                          (        1        )            
wherein f is a focus length of the objective lens, a is a diameter of the objective lens, xcex is a wavelength of the beam, and NA is the numerical aperture of the objective lens. It can be seen from the equation (1) that a position value(x) of the sub-beam is dependent only upon the numerical aperture NA and a wavelength(xcex) of the beam. For instance, when a wavelength(xcex) of the beam is 650 nm and the numerical aperture NA of the objective lens is 0.6, X becomes equal to 0.54 xcexcm.
In order to eliminate a cross talk component and optimally taking advantage of the sub-beam, however, a position value(x) of the sub-beam must be controlled in accordance with a track pitch P of the optical disc. In other words, since the track pitches P defined at each disc are different from each other even when the disc belongs to a disc series to which the same wavelength and the same numerical aperture are applicable, a cross talk component can not be optimally eliminated until a position value(x) of the sub-beam is adaptively controlled in accordance with the track pitch P. In the optical pickup apparatus as shown in FIG. 2, however, a wavelength(xcex) of the beam or the numerical aperture NA of the objective lens must be controlled so as to control a position value(x) of the sub-beam in correspondence with a track pitch of the disc, so that a degree of freedom for its design is deteriorated. Thus, it is necessary to provide an optical pickup apparatus that is capable of accurately locating the sub-beam at such a position that a cross talk can be optimally eliminated in accordance with the track pitch P.
Accordingly, it is an object of the present invention to provide an optical pickup apparatus that is capable of accurately locating a sub-beam at such a position that a crosstalk component can be effectively eliminated.
Further object of the present invention is to provide an optical pickup apparatus that is capable of eliminating a crosstalk component in compliance with various track pitches easily.
In order to achieve these and other objects of the invention, an optical pickup apparatus according to one aspect of the present invention produces a sub-beam for eliminating a crosstalk component by utilizing a polarizing phase plate which includes at least two phase zones with a different phase at each side on a basis of the center thereof.